The invention relates to an ion implantation process.
According to one such process, as is known from "Solid-State Electronics" (1972) Vol. 15, pp. 165 to 175, a substrate region arranged superficially in a semiconductor body, and by employing an oxide mask produced photolithographically with the aid of a resist mask, is produced by the implantation of ions which are subsequently diffused deeper into the substrate region. Moreover, in this process, oxide masks are exclusively used for limiting the region areas for the ion implantation.
In processes in which the doping material is deposited upon a plurality of differently doped regions by way of ion implantation, the exclusive use of oxide masks has been customary ever since. See, for example, "IEEE Journal of Solid-State Circuits", Vol. SC 14, No. 2 (Apr. 1979), pp. 312 through 318.
The exclusive use of oxide masks has the disadvantage that after each implantation, it is necessary to form a new oxide layer, especially in the case of a following rediffusion. Although the exclusive use of oxide masks is customary, it is known from DE-AS No. 23 41 154 to use a photoresist mask for ion implantation. Of course, with this process the photoresist mask is deposited exclusively on to an insulating layer covering the substrate surface and not, even partially, on to the substrate surface.
Experts were of the opinion that photoresist masks deposited at least partially directly on to the substrate surface were not very suitable for limiting region areas during ion implantation, because the photoresist, when deposited directly on to the substrate surface, appeared unsuitable for serving as the only masking against implantation, especially in the case of higher dosages.